Recently, measurements of concentrations of fine particulate matter in the air such as PM2.5 are widely performed. The mass of particles included in gas per unit volume is used as the unit of a concentration of particles in gas. This particle concentration is called a mass concentration. In an example of a standard method of measuring the mass concentration of PM2.5, particles in gas are collected with a filter and their mass is measured (see Japanese National Publication of International Patent Application No. 11-502303, for example). A beta-ray attenuation method is also available as a mass concentration measurement method by which automatic measurement is possible. Concentrations obtained in the filter sampling method and beta-ray attenuation method are mass concentrations. At present, PM2.5 concentrations are generally indicated as mass concentrations. As a simple method, a scattered light detection method is available in which particles in gas are illuminated with light and the number of particles in the gas is measured according to the resulting scattered light.
Japanese Laid-open Patent Publication No. 8-15122 is another example of related art.